The present invention relates to a progressive power lens used for correction of presbyopia.
The progressive power lenses for correction of presbyopia are developed and widely used. When the progressive power lens is designed and evaluated, the progressive power lens is often considered to have three portions: a distance portion for distance vision, a middle portion for middle vision and a near portion for near vision. Since refractive power within the middle portion changes continuously, the middle portion is referred to as a progressive area.
In general, clear definitions concerning object distances for the above three portions are not given. Therefore, in this specification, the term “distance portion” means a portion of a lens (i.e., a portion in a field of view of the lens) for viewing objects at relatively long distances, the term “near portion” means a portion of the lens for viewing objects at relatively short distances, and the term “middle portion” means a portion of the lens for viewing objects at middle distances between the relatively long distances and the relatively short distances.
In general, points on the lens for measuring vertex diopter within the distance portion and the near portion are referred to as a distance reference point and a near reference point, respectively. Although the distance reference point and the near reference point are often schematically illustrated as an area having a circular form or an elliptical form, in this specification, the distance reference point and the near reference point are considered as a center point of the circular area or the elliptical area.
Further, in this specification, a collection of points of intersection of a line of sight and a surface of the lens when a wearer (a person wearing spectacles) moves his/her eyes vertically is referred to as main meridian. The main meridian is a curved line passing through the distance reference point and the near reference point.
Since, in a case where objects at short distances are observed, a pupillary distance becomes shorter in comparison with a case where objects at long distances are observed, the progressive power lens is designed such that aberration distribution is bilaterally asymmetrical and the main meridian is shifted to a nose side from a center section through a lower area of the lens with respect to a reference line. The tern “reference lines” means a line extending in a vertical direction and passing through a center of the distance portion. In this specification, “upper”, “lower”, “horizontal”, “vertical”, and etc. represent directions with reference to a situation where the lens is wore by the wearer.
It is almost impossible to widen a clear vision area from the distance portion through the near portion. In this specification, the term “clear vision area” means an area through which the wearer can view objects without feeling distortion or blurring of an image. In many cases, the clear vision area is defined as an area in which transmission astigmatism is less than 0.5 or less than 1.0 [D: Diopter].
In general, a width of the clear vision area in the middle portion tends to be narrower in comparison with widths of the clear vision areas in the distance portion and the near portion. For this reason, the progressive power lenses are generally categorized into several groups depending on which portion of the lens has the maximum clear vision area.
A lens configured to have the largest possible clear vision areas both in the distance portion and the near portion and to have the middle portion with narrow clear vision area is referred to as a general-purpose progressive power lens. A progressive power lens for long/short distances belongs to the general-purpose progressive power lens.
With regard to the general-purpose progressive power lens, there is a problem that the wearer feels a so-called swinging of images due to distortion of an image which is caused by aberration in the middle portion and in particular in side areas within the middle portion when the wearer moves his/her line of sight relatively fast by shaking his/her head vertically or horizontally. The swinging of images causes uncomfortable feelings to the wearer. To solve this problem, it is required to keep astigmatism at a low level on the periphery of the middle portion.
For this reason, a progressive power lens for middle/short distances having larger clear vision area in the middle portion than that of the general-purpose progressive power lens has been proposed. The progressive power lens for middle/short distances is configured such that a rate of change in refractive power in a vertical direction within the middle portion is kept low by enlarging upward the size of the progressive area (i.e., the middle portion). Consequently, aberration in the side areas of the middle portion is reduced. Also, the progressive power lens for middle/short distances is configured to make the width of the clear vision area in the middle portion wider.
Although the above design of the progressive power lens for middle/short distances sacrifices a size of the clear vision area in the distance portion, the swinging of images can be suppressed and therefore the progressive power lens exhibits outstanding performance for indoor use in which the progressive power lens is mainly used for middle vision and near vision.
A progressive power lens for short distance has also been proposed. The progressive power lens for short distance is designed such that the clear vision area in a lower side within the middle portion and in the near portion becomes wider by narrowing the clear vision area in the distance portion and in an upper area within the middle portion. With this configuration, outstanding optical performance for work where the wearer is required to view objects at short distances is attained. It should be noted that, in the case of the progressive lens for short distance, the distance portion of the lens is a portion through which objects at relatively long distance (i.e., at distance of 1 through 2 meter) are viewed.
Points of interest of conventional lens design and evaluation of the progressive power lens were optical performance concerning a progressive power surface (hereafter, referred to as surface performance). That is, a dioptric power error and astigmatism each of which indicates the optical performance of a lens surface are calculated according to a difference between maximum principal curvature and minimum principal curvature of the progressive power surface and average values of the maximum principal curvature and the minimum principal curvature.
In addition, categorization of progressive power lenses (i.e., to categorize progressive power lenses as the lens for long/short distances, the lens for middle/short distances or the lens for short distance) was made according to aberration distribution of a lens surface.
However, optical performance of a spectacle lens should be evaluated by analyzing light beams passing through various points on the spectacle lens because the wearer feels the spectacle lens suitable if the optical performance evaluated by the light beams passing through the points on the spectacle lens is excellent. In this specification, the optical performance evaluated based on the light beams passing through the points on the spectacle lens is referred to as transmission performance.
It should be noted that the transmission performance differs from the surface performance and the difference between the transmission performance and the surface performance becomes greater particularly within peripheral regions of the lens. In Japanese Provisional Publication No. HEI11-125580 previously filed by the assignee of the present invention, the transmission performance and the evaluation method thereof are discussed in detail.
Meanwhile, the spectacle lens is required to have aesthetic appearance in a condition where the spectacles are worn by the wearer. In general, a thin spectacle lens is desired in terms of aesthetics and manufacturability. Further, since the spectacles are consists of a pair of lenses and a spectacle frame, the aesthetic appearance of the spectacle lens should be evaluated considering a balance between right and left lenses.
In this point of view, front surfaces (object side surfaces) of the right lens and the left lens are equal to each other and rear surfaces (eye side surfaces) of the right lens and the left lens are equal to each other when refractive power required of the right and left lenses are the same.
On the contrary, when a difference between the refractive power required of the right lens and the left lens is relatively great, if the right and left lenses are individually designed, forms of the light and left lens becomes different from each other. In this case, the balance between the right and left lenses is lost and therefore aesthetic appearance of the spectacles is damaged. Therefore, to provide spectacles having aesthetic appearance, it is preferable that outer surfaces of the right and left lenses are equal to each other.
However, if the right and left lenses are designed considering only forms of the lenses so that base curves of the right and left lenses are matched with each other, aberration increases because in general a form of the lens which minimizes aberration with reference to required refractive power is limited. Consequently, optical performance deteriorates. Therefore, it is required to design spectacle lens considering a balance between a form of the lens and optical performance of the lens. Thus, conventionally, only a single kind of lens having a certain base curve with reference to a certain refractive power has been designed.
At this point of view, three kind of progressive power lenses and a manufacturing method are proposed in Japanese Provisional Publications No. HEI9-90291 (document 1), 2001-318344 (document 2), 2001-318345 (document 3) and 2002-122824 (document 4).
The progressive power lens disclosed in the document 1 is designed by cut-and-try methods to achieve suitable transmission performance. Although in the document 1 some guidelines about a lens design to improve distribution of dioptric power are explained, guidelines concerning a lens design for improving astigmatism which has a great effect on optical performance of the lens are not discussed at all. From a distribution map of transmission astigmatism indicated in the document 1, it is understood that a sufficiently wide clear vision area is not achieved.
Each of progressive power lenses disclosed in the documents 2 and 3 belongs to a certain progressive power lens family which is designed to have constant optical performance even if a base curve of the lens changes according to spherical refractive power required by the wearer.
Although the progressive power lens in each of the documents 2 and 3 is designed to primarily achieve an improvement in optical performance within the distance portion and uniformity of a series of progressive power lenses, concrete suggestion with regard to a lens design for improving distribution of astigmatism of the whole lens is not discussed at all. In addition, in the documents 2 and 3, to match a base curve of a left lens to that of a right lens so as to improve aesthetic appearance of the spectacles is not discussed at all.
A manufacturing method disclosed in the document 4 achieves a progressive power lens having a certain spherical refractive power within the distance portion for a plurality of kinds of base curves. Further, according to the manufacturing method of document 4, deterioration of optical performance of each lens can be reduced to a minimum when an identical base curve is adopted by right and left lenses having different dioptric power. However, the optimization of optical performance of the lens discussed in the document 4 only relates to reducing aberration of each lens. To equate distribution of transmission aberrations of a plurality of lenses having different base curves is not discussed in the document 4 at all.
Advantages of achieving uniform distribution of transmission aberrations for lenses having different base curves and/or different spherical refractive power within the distance portions are as follows.
One of the advantages is that the wearer can have natural view without having different visions through right and left lenses even if base curves of the right and left lenses are designed to be equal to each other to keep an appearance of the lens excellent in a condition where dioptric power in the right lens and the left lens are different from each other.
Further, if the distribution of transmission aberration can be kept substantially constant among the lenses having different spherical dioptric power, to equate base curves of the lenses can be attained within wide range of the lenses. Consequently, the range of choices of the lenses for the wearer may be extended. However, a progressive power lens family having a uniform aberration balance for different base curves and for different spherical dioptric power has not been provided.